Automotive Engine Performance

Pressure Sensors

18-02 Classify different types of pressure sensors.

Pressure sensors on an ICE convert fluid or air pressure into an electrical signal so that the PCM can determine what is happening in the engine (FIGURE 18-12). Verifying the pressures created by the mechanical and electrical systems on the engine allows the PCM to fine-tune the engine operation. These sensors are very sensitive to the changes of pressures they are reading, so if the pressure deviates from specification, these switches may be damaged. Like other electrical components, pressure sensors are susceptible to excessive pressure and to fluid intrusion when exposed to liquids on their electrical side. This intrusion could come from the high pressures they are exposed to or from an external source. When diagnosing these types of components, make sure to verify that it is not being affected by an outside source.

FIGURE 18-12 A pressure sensor takes a reading of the gas or liquid that is creating the pressure on the engine and then converts it to an electrical signal that the PCM can understand.

Fuel Rail Pressure Sensor

As technology has progressed, the need for a fuel pump to be operating continuously has waned, minimizing the operation time of the fuel pump increases the life of that component. To modulate a fuel pump it requires a module to turn it on and off. To allow the module to determine when to cycle the fuel pump and thus effectively control the fuel pump, it needs to detect the fuel pressure input. The fuel rail pressure (FRP) sensor provides a signal to the fuel pump module to allow it to make a decision on how long to turn on the fuel pump (FIGURE 18-13). The sensor is a three-wire transducer that has a 5 Vref (voltage reference) from the PCM, a low or ground wire, and a signal wire that is sent back to the fuel pump module. As the fuel pressure changes, the signal to both the PCM and the fuel pump control module adjusts to better reflect what is physically happening in the fuel rail. This signal will fall into a certain area in the module’s programming, which will tell the module how long to keep the fuel pump on to increase or decrease the fuel pressure as needed.

FIGURE 18-13 The FRP sensor tells the PCM how much fuel is left in the fuel rail from the injection events. This measurement can also be used to verify that the fuel pump is operating at the correct level.

Engine Oil Pressure Sensor

The mechanical systems on the engine should be monitored by the PCM so that if a failure or a potential failure is happening, the PCM can intervene. A major system that must be monitored is the oiling system. The engine oil pressure (EOP) sensor is used to change a fluid pressure into an electrical signal that the PCM can understand (FIGURE 18-14). The sensor has a rubber diaphragm in the bottom that is connected to a movable switch that changes the resistance on the switch as the pressure is changed. This changes the voltage output of the switch that goes to the PCM and instrument panel to operate the gauge. The PCM verifies that engine is making oil pressure to help the instrument panel cluster (IPC) or the body control module(BCM) by telling either to turn on the light or move the gauge to the proper location. The sensor has a 5 Vref wire, a low or ground wire, and a signal wire. The sensor changes the output of the signal wire based on the amount of oil pressure that is moving the diaphragm.

FIGURE 18-14 The oil pressure sending unit is installed in an oil gallery so that it has an unobstructed oil pressure feed. The sensor can also be used for various modules on the vehicle to minimize the need for extra sensors.

Manifold Absolute Pressure and Barometric Pressure Sensors

The manifold absolute pressure (MAP) sensor is used to determine the load on the engine. The sensor is made with a piezoresistive crystal impregnated on a silicon board (FIGURE 18-15). This sensor uses a 5 Vref, low or ground wire, and a signal wire that goes back to the PCM with the proper signal generated by the sensor. The crystal is exposed to a perfect vacuum on one side of the circuit board and to engine vacuum on the opposite side. Because of the differences in pressure the crystal changes the resistance which alters the 5 Vref that is being fed into the sensor. Changing the resistance also changes the voltage that is sent back to the PCM. These types of sensors must have a good vacuum source to operate properly, so when diagnosing them as a failed component, verify that the sensor has the proper vacuum reading. The barometric pressure (baro) sensor is sometimes integrated with the MAP sensor since they work in a similar way. The baro sensor is used to measure the atmospheric pressure that the engine is operating in (FIGURE 18-16). This is necessary because combustion is affected by the pressure present in the air around the engine. This sensor is vented directly to the atmosphere so that it can get an unobstructed reading of what is happening outside of the engine. The PCM uses the combination of both of these sensors to help it deliver the proper fuel mixture to the engine.

FIGURE 18-15 The MAP sensor uses engine vacuum to determine the load on the engine, which is used by the PCM to change the fuel ratio and ignition timing.

FIGURE 18-16 Because the vehicle may be operated in different areas of the world, the needs of the engine to support the combustion process may need to change—changes that must be updated by the PCM to sustain combustion. The baro feature of the MAP sensor allows the PCM to determine the density of the air.

Knock Sensor

The knock sensor is used on an engine to monitor for detrimental engine events (FIGURE 18-17). When an event happens in the engine that could potentially damage components, it usually makes a loud noise that then can be picked up by the knock sensor. These noises, called pinging, are usually caused by the ignition system. When such an event occurs, the sensor generates a voltage that the PCM receives, and it then retards the ignition timing until the pinging goes away. Once the noise goes away, the PCM advances the timing again and tries to operate the engine as normal. If the noise continues, the ignition will remain retarded and a check engine light will be illuminated. The sensor has only one wire to it and a piezoelectric crystal that makes voltage when it is vibrated (FIGURE 18-18). Once this voltage is produced, the PCM receives the signal and it continues to try to save the engine from internal damage. The knock sensor is usually on the engine block near a coolant jacket. Either it is bolted to the block or it screws into the block. The body of the knock sensor acts as a ground, and the power comes out of the wire attached to the sensor.

FIGURE 18-18 The knock sensor uses a piezoelectric crystal to generate a signal when an event happens. The vibrations of the event excite the crystal, which then generates a voltage that the PCM can read.

Mass Airflow Sensor

The MAF sensor is used to measure the amount of air that is entering the engine’s intake manifold. This is necessary to determine the amount of fuel needed to operate the engine efficiently (FIGURE 18-19). This sensor depends on a sealed system that will not allow stray air to enter the intake after the MAF sensor. When diagnosing a MAF fault, pay special attention to where the MAF is located: If it is located before the intake snorkel, check for a leak in the intake snorkel. Also check the tightness of the snorkel to make sure that it will not move (FIGURE 18-20).

FIGURE 18-19 The MAF sensor measures the incoming air to give the PCM an indication of the amount of air being sucked into the engine. This allows the PCM to calibrate the fuel-injection system to create an efficient mixture.

FIGURE 18-20 Outside air that is drawn in after the MAF sensor can cause the sensor to read incorrectly, and the PCM will then not be able to account for the extra airflow, which can cause a misfire.

The MAF sensor could be one of a few different types. A hot wire–type MAF sensor uses a sensing wire that is heated up by sending electricity through the sensor. The element has a set resistance, and the PCM knows the input voltage into the sensor as well as what the output should be. As the airflow flows over the element, it cools the element at a certain rate (FIGURE 18-21). The output of the sensor changes based on how cool the element becomes, and the PCM can interpret that change in voltage with a specific amount of airflow. Once the PCM has this reading, it can determine the correct amount of fuel to make the engine operate efficiently. The sensing wire is very thin, and it must be exposed to the incoming air from the filter. Exposure to the air can cause debris to contaminate the wire, air filters to disintegrate, or not using an air filter in the application can all lead to premature sensor failure. (FIGURE 18-22). The output of this type of sensor is voltage based, which can adjust quickly to the changes in airflow. This sensor measures air mass, not volume. A similar MAF sensor used by General Motors and other OEMs integrates another wire that measures ambient air temperature, which provides the sensor with a reference signal to assist with more precise airflow measurement.

FIGURE 18-21 The hot wire MAF sensor measures the temperature of the wire and how quickly it cools when air passes over it. It determines the amount of airflow by calculating the amount of resistance change in the wire. This is one of the most common types of MAF sensors used.

FIGURE 18-22 Contamination on the sensor can cause a false reading, which will throw the fuel mixture out of specification and can cause running issues.

The vane airflow (VAF) sensor uses a spring-loaded door attached to a potentiometer to measure the volume of air entering the engine (FIGURE 18-23). As the airflow moves the door, it moves the potentiometer to send a signal back to the PCM (FIGURE 18-24). The PCM can interpret the mass of airflow that is entering the engine so that it can adjust the mixture to accommodate the increase or decrease of air. Because of the intake pulses and varying rpm of the engine, this sensor has a dampening chamber so that the pulses will not overly affect the vane door, which could cause the PCM to adjust for no reason. These sensors have a lot of moving components that can become broken or stuck over time, so they must be examined every time that there is a running complaint.

FIGURE 18-23 A vane airflow sensor uses a door and a potentiometer to generate a signal that the PCM can understand.

FIGURE 18-24 If the door becomes stuck or the potentiometer becomes skewed, the fuel delivery can become an issue that will cause a running problem.

The Karman vortex MAF sensor is used to a portion of the sensor sticking out in the intake tract of the sensor to disrupt the airflow (FIGURE 18-25). This disrupted airflow is then measured by a sensor that measures pressure, or it is pushed against a mirror, which interrupts a light beam projecting throughout the sensor to generate a frequency. The PCM then interprets this frequency to determine the air velocity going through the sensor. This velocity is then used to calculate the amount of air entering the engine. These type of sensors are used primarily on vehicles imported from Asia.